Extendable leg assemblies such as included in tripods and the like generally include an inner leg coaxial with and supported for telescoping movement within an end of an outer leg. A hand-operable lock mechanism is usually employed to prevent undesired telescoping movement between the inner and outer legs. One common form of such a lock comprises a collet built into the outer leg. By turning the collet in one direction it is tightened against the inner leg to prevent axial movement thereof. The heavier the load of the leg assembly, the tighter the collet must be to prevent axial movement between the legs. For any degree of collet tightness however, there is a critical load which will cause the inner leg to slip within the leg.
Another form of lock for an extendable leg assembly is disclosed in U.S. Pat. No. 3,480,247 issued Nov. 25, 1969, for "Self-locking Adjustable Stanchion." While the stanchion is self-locking, axial movement of a collet is required for release of the lock. Also, as disclosed, the lock is gravity sensitive in that vertical movement of the stanchion can cause accidental lock with falling movement of a lock ring within the lock assembly.